1. Field of the Invention
The present invention relates to the receiving technology, and it particularly relates to a receiving method and apparatus for detecting the timing of a received packet signal, and a communication system utilizing the same.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize the high-speed data transmission and are robust in the multipath environment. This OFDM modulation scheme has been applied to the wireless standards for wireless Local Area Network (LAN), such as IEEE802.11a/g and HIPERLAN/2. The packet signals in such a wireless LAN are generally transferred via a time-varying channel environment and are also subject to the effect of frequency selective fading. Hence, a receiving apparatus generally carries out the channel estimation dynamically.
In order for the receiving apparatus to carry out the channel estimation, two kinds of known signals are provided within a packet signal. One is the known signal, provided for all carriers in the beginning of the burst signal, which is the so-called preamble or training signal. The other one is the known signal, provided for part of carriers in the data area of the packet signal, which is the so-called pilot signal (See Reference (1) in the following Related Art List, for instance).
Related Art List
    (1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems”, IEEE Transactions on broadcasting, vol. 48, No. 3, pp. 223-229, September 2002.
In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the directional patterns of antennas are controlled by controlling the amplitude and phase of signals, to be processed, in a plurality of antennas, respectively. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and a plurality of packet signals to be transmitted in parallel are set (hereinafter, each of data to be transmitted in parallel in the packet signal is called “stream”). That is, streams up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates.
Moreover, combining such a MIMO system with the OFDM modulation scheme results in a higher data transmission rate. A known signal defined in a communication system which is not compatible with a MIMO system (hereinafter referred to “legacy system”) is assigned to a leading part of a packet signal in such a MIMO system. Accordingly, a receiving apparatus of a legacy system recognizes the leading part of a packet signal and can recognize that it is the packet signal which is not to be received by the legacy system. As a result, the receiving apparatus of a legacy system stops receiving the rest of parts in the packet signal, thus reducing the power consumption. In the packet signal in a MIMO system, a known signal defined in a MIMO system is also assigned posterior to the known signal of a legacy system. The reason for this is as follows. A plurality of streams are transmitted in a MIMO system, whereas a single stream is transmitted in a legacy system. Hence, the signal strengths in the receiving apparatus differ, and the receiving apparatus sets the gain of amplifier according to the signal strengths.
Based on the known signals, the receiving apparatus detects the timing, detects the frequency offset, derives the receiving weight vectors, estimates the channel and so forth. Here, the detection of timing will be discussed. When the receiving apparatus receives packet signals, it detects the presence of a packet signal and at the same time detects the timing. Accordingly, the receiving apparatus performs correlation processing on between the received packet signal and a legacy known signal so as to detect the peak of correlation values. In so doing, a packet signal whose delay time is long shall also be received in order to raise the accuracy in detecting the presence of packet signal and therefore the range over which the addition is executed (hereinafter this range will be referred to as “correlation window”) needs to be broad. However, when the correlation widow is broad, the timing in which a plurality of delayed wave components contained in the received packet signal are combined is detected. Thus, timing in which the effect of a plurality of delayed wave components contained in the received packet are taken into account is no longer detected. As a result, the highly accurate timing detection cannot be executed.